How to Read S-Parameters Like a Book or Tapping Into Some Of The Information Buried Inside S- Parameter Black Box Models

Transcription

1 Slide -1 Bogatin Enterprises and LeCroy Corp No Myths Allowed Webinar Time before start: How to Read S-Parameters Like a Book or Tapping Into Some Of The Information Buried Inside S- Parameter Black Box Models Dr. Eric Bogatin, Signal Integrity Evangelist, Bogatin Enterprises a LeCroy Company Downloaded handouts from, recent publications: PPT-NMA-855 Fall 211 Slide -2 Overview S-parameters as a black box behavioral model Root cause analysis: a powerful methodology Four common patterns and their possible root cause A Pop Quiz

2 Slide -3 For More Information Recent Publications My Blog: What I learned this month Class schedule Boston: Sept Published by Prentice Hall, 29 Hyderabad: Oct 1-11 Bangalore: Oct Singapore: Oct 18 Penang: Oct 2 Shah Alam: Oct 21 San Jose: Nov 8-1 Slide -4 Hands on Labs in All Bogatin Enterprises Classes (simulation software and lab exercises provided) Quite Universal Circuit Simulator (QUCS) Circuit Simulator: transient, frequency domain, S-parameter LeCroy s SI Studio S21 S11 S-parameter viewer Simulate eye diagrams from channel S- parameters Serial data analysis

3 Slide -5 A Special Thank you for all Webinar Attendees 1% discount to any Boston or San Jose, Bogatin Enterprises class: SPSI, Sept 28, 211, Boston DPD, Sept 29, 211, Boston EPSI, Nov 8-9, 211, San Jose SPSI, Nov 1, 211, San Jose Use discount code NMA119A when registering online at Slide -6 The Real World: Interconnects are Not Transparent TX RX Driver Package Board Backplane Board Package Receiver 1. We use S-parameters to describe the behavior of the interconnects 2. We can answer some why questions from the S-parameters

4 Slide -7 Characterize an Interconnect by How Precision Reference Signals Scatter Off a DUT Time Domain t Incident Wave Transmitted wave t Frequency Domain TDR S11 parameter = waveform out from a port waveform in to a port TDT S21 Slide -8 What are S-Parameters? A collection of scattered responses (at every frequency): Reflected sine waves S 11 = sine wave out from port 1 sine wave into port 1 Magnitude and Phase or Transmitted sine waves S 21 = sine wave out from port sine wave into port 1 2 Real, and Imaginary Stored in a special format: Touchstone file.s1p: scattered data from 1 port.s2p: scattered data from 2 ports Freq units are MHz S-parameters db and degrees Reference Z 5 ohm

5 Slide -9 System Simulation with S-Parameter Behavioral Black Box Models TL1 TL3 TL5 V1 U1 1 2 CMOS,3.3V,FAST J2 Port1 Port2 Port3 Port4 m1_1234.s4p 83.5 ohms ps 3. in Stackup TL2 TOP BO ohms ps 3. in Coupled Stackup TL4 J3 Port1 Port2 Port3 Port4 m1_1234.s4p 83.5 ohms ps 3. in Stackup TL6 J4 Port1 Port2 Port3 Port4 m1_1234.s4p U2 1 2 CMOS,3.3V,FAST 83.5 ohms ps 3. in Stackup 83.5 ohms ps 3. in Stackup 83.5 ohms ps 3. in Stackup Differential Response, db Differential Response freq, GHz Differential Response, db freq, GHz freq, GHz Turn S-parameter Behavioral Model into a SPICE compatible model using pole-zero model of S- parameters which any SPICE can use: 5 RX broad band SPICE : Simbeor, HyperLynx, ADS, Sigrity, SiSoft Slide -1 Opening the Lid to the Black Box What treats lay within?

6 Slide -11 Fastest Way to Solve a Problem is to Identify its Root Cause If you have the wrong root cause, you will only fix the problem by luck Slide -12 Don t Think Frequency or Time Domain, Think Frequency AND Time Domain Single ended S-parameters Frequency Domain Measurement Differential S-parameters Circuit simulation Electromagnetic simulation Single ended T-parameters Time Domain Differential T-parameters

7 Slide Port S-Parameters Magnitude and phase Detector Applies to single-ended and differential S-parameters V source ~ 5Ω Transparent interconnect: S11: large, negative db S21: small, negative db Z = 5Ω DUT S11: Return loss Z = 5Ω It is the reflected signal Impedance mismatch from 5 ohms throughout the interconnect A little about losses S21: Insertion loss It is the transmitted signal Impedance mismatches throughout the interconnect Losses 5Ω magnitude/ phase detector Slide -14 Four Important Patterns in S11, S21 Ripples in S11, sometimes in S21: reflections Monotonic drop in S21: losses Broad dips: ¼ wave stub resonances Sharp dips (hi Q), coupling to resonances Insertion loss return loss Insertion loss

8 Slide -15 How Reflections Result in Return, Insertion Loss Ripples min S11 Z < 5 Ohms 5 Ω 5 Ω S21 When Len << ¼ λ max S21 At low frequency ALL interconnects are transparent S11 When Len << ¼ λ Reflections from front and back, 18 deg out of phase No net reflection, all transmitted waves in phase and add S11 large negative db, S21 nearly db Slide -16 When Len = ¼ λ, S21 Lowest, S11 Highest Z < 5 Ohms S21 min S11 max S11 When Len << ¼ λ When Len = 1/4 λ ¼ + + max S21 ¼ ¾ min S21 When Len = ( x n + ¼ ) λ Max S11, min S21 S11

9 Slide -17 When Z 5 Ohms, Multiple Reflections From The Terminations Cause Ripples Z < 5 Ohms S21 min S11 max S11 min S11 1 When Len << ¼ λ When Len = 1/4 λ + ¼ + When Len = λ max S21 ¾ ¼ min S21 max S21 When Len = n x λ Reflected waves from front and back subtract Minimum reflected signal Transmitted waves all in phase, S21 max As frequency increases, insertion, return loss increase, decrease Longer distance between reflections, shorter the frequency between high and low Larger the impedance difference, the larger the modulation S11 Slide -18 Attenuation and Insertion Loss In real interconnects, amplitude drops off exponentially with distance V d α α d 2 out( ) in in nepers/len V d = V e = V 1 db/len out S21= Vin V out S21[ in db] = 2 x log = α [ db / in] x d= attenuation Vin S21 is attenuation when terminations are matched and there is no coupling out of the transmission line d

10 Slide -19 Estimating Attenuation per Length in 5 Ohm Interconnects 1 attenlen = f x f x Df x Dk w atten Len GHz ~ ( 2.3 x Df x Dk ) ~.1dB / inch / GHz db/inch w = line width in mils f in GHz Dk = dielectric constant Df = dissipation factor Measured attenuation from two different transmission lines w = 5 mils Dk = 4 Df =.2 Simple estimate matches measured behavior ok Never sign off on a design based on a rough estimate Slide -2 The ¼ Wave Stub Resonance TD, Len stub Len TD= 6 in nsec 2 x TD When 2 x TD = cycle, minimum received signal TD = ¼ cycle: the quarter wave resonance 1 1 TD= 4 f res f res 1 1 = 4 TD 1.5 = Len f in GHz Len in inches Example: Len =.5 inch, f res = 3 GHz

11 Slide -21 ¼ Wave Resonance in the Frequency Domain Len stub Stub length =.5 inches f res 1 1 = 4 TD 1.5 = Len Stub.5 inches long f res = 3 GHz Where does the energy go? S21 S11 If Nyquist = 3 GHz, no eye at BR = 6 Gbps Slide -22 Measured Insertion Loss with and without Via Stub in a ¼ inch Thick Backplane No via stub in the channel.2 inch via stub 7.5 GHz f res = = = 7.5 GHz Len.2

12 Slide -23 Coupling to Hi-Q Resonances What are hi-q resonators? Any floating metal (copper fill) Plane cavities Slide -24 Via to Cavity Coupling in a 4 Layer Board 3.25 inches.8 inches inches.8 inches 12 GHz f res = = Dk 2 x Len 3 GHz Len S21 wo return vias Plane resonances expected: Len = 3.25 in f res =.92 GHz Len = in f res = 2.5 GHz Len =.8 in f res = 3.75 GHz

13 Slide -25 Four Important Patterns in S11, S21 Ripples in S11, sometimes in S21: reflections Monotonic drop in S21: losses Broad dips: ¼ wave stub resonances Sharp dips (hi Q), coupling to resonances Insertion loss return loss Insertion loss Slide -26 Pop Quiz: What Causes the Higher Loss? Measured SDD21 for 2 differential pairs, up to 1 GHz Same measurement, but up to 2 GHz Dip at 14 GHz Looks like bottom (blue) line has more SDD21 than top (red) line. What causes the higher attenuation? -Conductor loss from surface roughness? - Poor copper plating - High Df in one layer -??? What causes the large dip? - Stub resonance? - Mode conversion? - Resonant coupling to other structures? - Bloch waves and glass weave? - Dielectric absorption resonance? Data courtesy of Bob Haller, Enterasys Which S-parameters might have the answers?

14 Slide -27 Could it Be Mode Conversion? SDD21 SDD11 SCD11 Mode conversion terms: SCD11, SCD21 SCD21 SDD11 shows reflected energy SCD11 shows mode conversion reflected SCD21 shows mode conversion transmitted Slide -28 Get in the habit of looking inside S- parameters Summary You can observe a lot by watching, Yogi Berra Look for the four patterns Ripples Monotonic drop Broad dips Sharp dips This has been just a tip of the iceberg look at data mining S- parameters

15 Slide -29 Slide -3 For More Information Recent Publications My Blog: What I learned this month Class schedule Boston: Sept Published by Prentice Hall, 29 Classes with Hands on Labs: Quite Universal Circuit Simulator (QUCS) LeCroy s SI Studio S21 S11 Hyderabad: Oct 1-11 Bangalore: Oct Singapore: Oct 18 Penang: Oct 2 Shah Alam: Oct 21 San Jose: Nov 8-1